Wireline logging has been done for many years to enhance recovery of oil and gas deposits. In borehole logging, one method of making measurements underground includes attaching one or more tools to a cable connected to a surface system. The tools are then lowered into a borehole by the cable and then drawn back to the surface (“logged”) through the borehole while taking measurements. The cable often includes multiple conductors, such as a 7-conductor “hepta-cable.” The conductors of the cable provide power to the tools from the surface and a route for electrical signals to be passed between the tools and the surface system. The signals may be, for example, tool control signals that pass from the surface system to the tools, and tool operation signals and data which pass from the tools to the surface system.
A common telemetry system for facilitating communication between the surface system and the tools may include a telemetry module (TM) at the surface, the cable, and a downhole telemetry cartridge (TC) at the head of a string of tools. Each downhole tool will typically include a downhole toolbus interface (BI) for communicating with the TC via a downhole toolbus (TB). This telemetry system is configured to allow data flows in two directions: from the TM to the tools and from the tools to the TM. Communications from the subsurface up the borehole to the TM are called an “uplink”. Communications from the TM down the borehole to the subsurface are termed a “downlink”.
In a typical telemetry system, each tool sends its data to the downhole telemetry cartridge through the toolbus. The telemetry cartridge then sends the data to the telemetry module, usually via a wireline cable. While this configuration simplifies the downhole telemetry, it requires that all data be communicated to the surface. Thus, in the situations where it is desired to communicate data or a signal from one downhole tool to another downhole tool, the typical telemetry configuration requires that the data be sent from the first tool to the TC, from which it is communicated via an uplink to the TM, then communicated from the TM via a downlink back to second tool via the TC. The time required for such up-and-down communication is inefficient, particularly in deep boreholes where the distance between the TC and TM can be large. A method for communicating downhole between borehole tools is needed. It would be advantageous if this communication method were compatible with conventional downhole tools designed to communicate with a downhole telemetry cartridge through a toolbus.